1. Field of the Invention
The present invention relates to an optical recording medium such as an optical disc, and more particularly to a write-once type compact disc on which information signals which meet the requirements of the International Compact Disc Standards can be recorded and from which information signals recorded thereon can be reproduced.
2. Discussion of Background
Development of write-once type compact discs provided with a recording function has recently become very active. The object of the development of such write-once type compact discs is to obtain compact discs from which recorded information can be reproduced by commercially available compact disc players. Therefore these compact discs must meet the requirements of the International Compact Disc Standards. In these Standards, the compacts are required to have a reflectivity as high as 70% or more. Many of these compact discs have a structure which includes a reflection layer made of a metal such as gold or aluminum.
Japanese Laid-Open Patent Application 2-87339 discloses an optical recording medium which comprises a substrate, a light-absorbing layer which is formed on the substrate by spin-coating a dye which absorbs a laser beam, and a reflection layer made of a metal overlaid on the light-absorbing layer. This optical recording medium satisfies the requirements of the International Compact Disc Standards, in particular with respect to various signal levels and the reflectivity, so that the reproduction of recorded information therefrom by a commercially available compact disc player is possible.
As mentioned above, the dye employed in this optical recording medium absorbs the laser beam. However, such a dye does not have a maximum absorption to the wavelength of the laser beam employed. If a dye having a maximum absorption to the wavelength of the laser beam is used, the absorption of the laser beam by the dye is so great that little light is reflected from the reflection layer provided on the back side of the light-absorbing layer. The result is that the requirement for the reflectivity of 70% or more cannot be met. For this reason, the dye employed in this optical recording medium is selected from dyes which do not have a maximum absorption with respect to the wavelength of the laser beam employed, and a high reflectivity is obtained by utilizing the effects of the multiple reflection interferences on the opposite sides of the dye-containing light-absorbing layer. Hereinafter this dye-containing light-absorbing layer is referred to as the dye layer. In order to obtain a high reflectivity by utilizing the interference effects, the complex index of refraction (n-ik) and the thickness of the dye layer must be optimized. The optimum values of the complex index of refraction (n-ik), and the thickness are respectively obtained when n.gtoreq.1.8, and k.ltoreq.0.3, and when the thickness is in the range of 100 nm to 200 nm.
However, when a dye layer with such a thickness is formed by the spin-coating method on a substrate in which pregrooves for tracking have been made, those pregrooves are completely filled with the dye layer, so the problem is created in that it is difficult to produce track error signals therefrom. This problem could be avoided by providing deeper grooves which cannot be completely filled with the dye layer. However, the production of a substrate with such deeper grooves is not easily accomplished by injection molding.
Furthermore, in order to produce the dye layer with the above-mentioned thickness by the spin coating method, it is necessary that a coating liquid for the spin coating contain the dye in high concentration. In order to attain this, the dyes employed in this method are required to have high solubility in the solvents for the coating liquid. The number and kind of dyes that can be employed in this method are extremely limited by this requirement.
In order to produce the recording medium at low cost, it is preferable that a recording layer be directly formed on a substrate made by injection molding, without interposing an undercoat layer between the substrate and the recording layer. However, polycarbonate resin and acrylic resin which are mostly employed for the substrate of the recording medium are not resistant to solvents, and the solvents that can be employed with such resins are strictly limited to alcohol solvents. Thus, this method has the problem that the selection of the dyes and solvents for the coating liquid is difficult.
There is a method of forming a recording layer by the spin coating method, and providing a reflection layer on the recording layer by the vacuum film formation method. The productivity of this method is not good because both a wet process for the spin coating and a dry process for the vacuum film formation method must be employed.